BEIRUT (AP) – Syrian opposition groups on Sunday rejected a U.N.-brokered peace plan for a political transition in Syria, calling it ambiguous and a waste of time and vowing not to negotiate with President Bashar Assad or members of his “murderous” regime.

The disappointed reaction underlined the seemingly intractable nature of the Syrian conflict, which this week saw some of the bloodiest violence since the start of the uprising against Assad’s regime in March 2011.

Activists said dozens of people were killed and wounded in a powerful explosion Saturday evening that hit a funeral procession in a suburb of the Syrian capital Damascus. Details of the blast in Zamalka remain murky, but amateur videos showed gruesome images of bodies, some with their limbs torn, lying on the ground as people walked about dazed in a cloud of smoke.